It’s the spookiest time of year again- time for costumes and creepy urban legends and candy and movies using gallons of fake blood. Who doesn’t love a good shiver? After all, for the most part, we modern humans have few of our ancestral fears left. No tiger or lion is going to spring out of the undergrowth at us while we’re waiting in line at the DMV.

In large part, this is because humans have systematically eradicated possible predators from every ecosystem we inhabit. Largely gone are the grey wolves, the cougars, the lions, the tigers, and even the sharks from their ancestral ranges. The amount of territory lost, if you look at it in a visual representation, is stunning.

With these threats (mostly) removed, we humans feel much safer. Perhaps too safe, judging by the amount of money we all spend each year to get scared in haunted houses or amusement park rides.

But by removing top predators, we don’t just ease our own fears. When impala stop seeing, hearing, and smelling evidence of lions and cheetahs, they too breathe a sigh of relief. And what happens next- forgive me- can be a little scary.

Two years ago, I wrote a piece on the current overpopulation of white-tailed deer in the United States. The effect that these deer have on forests cannot be understated: they eat new saplings and demolish the undergrowth, leaving hundreds of other species without food or homes. I argued then that to protect our forests, we need to somehow reduce the numbers of white-tailed deer.

It would be nice if this happened a little less often, too.

I still believe that we have to pull down the population of deer, but there are other factors we overlook that greatly affect how prey species interact with their environment. The number of healthy animals that carnivores actually manage to kill is surprisingly small compared to the number of animals left alone. Disease and famine are much more significant causes of death for most animals then getting snapped up by someone else. (The exception to this is when an invasive carnivore is abruptly introduced to an ecosystem.)

The chance for an individual (healthy) impala to be killed by a lion might statistically be quite low, but they wouldn’t survive as a species if they weren’t driven to change their behavior if they smelled a predator in the area. And that, by and large, is the main effect that predators have on prey, not the part where they eat them.

So how does fear change prey behavior? Numerous studies have examined this by either “muzzling” predators or simply leaving evidence of their existence and watching what the prey animals do. One recent study involved playing dog barks and growls to scare raccoons living on the Gulf Islands in British Columbia. According to the authors, the effects weren’t just immediate- they were large-scale and persisted over a month.

Raccoons used to have more predators on the islands until humans moved in (a common enough story), but now their main predators are domestic dogs. These island raccoons normally spend a lot of their time foraging for crabs along the seashore. However, once the researchers started playing dog calls, there was a dramatic shift: the raccoons started avoiding the shore.

This isn’t actually too surprising, of course. The shore is a very open and exposed place to be if you’re afraid of getting snacked on. What was surprising was how quickly this affected other species along the shoreline. The crabs that the raccoons would normally have preyed upon began to appear in much greater numbers. The crabs ate the food as a species of fish, which began to decrease in number as the crabs increased. And the numbers of the crabs’ own prey, a species of snails, started decreasing in number.

This is what’s known as a ‘trophic cascade,’ meaning it affected multiple levels of the ecosystem. No doubt there were other effects that the researchers didn’t measure, too: for example, perhaps the algae that the snails fed on also increased.

The fascinating thing about all this is that the raccoon population was in no way reduced or restricted. Their own fear was the only thing that changed all of these different animal populations. Scared animals, it turns out, spend less time eating in one spot, and eat less overall. Instead, they spend more time with their heads up, scanning for danger, and are more likely to bolt to a new position at small provocations. They also seek out areas that may not have the best food, but do have good cover, like forests or rocky outcroppings. They may even change their group composition, splitting from large spread-out herds to tight little knots of nervous animals. All this anxiety means that whatever food they eat, plants or crabs, isn’t totally decimated when they leave.

This sort of trophic cascade was exactly what researchers thought they had found in Yellowstone Park in 2004. Perhaps you’ve heard of this one before- there was a popular ‘wolves change the course of rivers’ video that was passed around a lot on social media a while back. Here’s the gist of it:

Like the white-tailed deer, the overpopulated elk in Yellowstone were having a dramatic effect on the growth of saplings, particularly along riverbanks. By removing the saplings, the elk caused greater erosion of the soil at the water’s edge, which caused the rivers to flow wider and slower. The video claims that when wolves returned, the elk immediately began avoiding the exposed riverbanks, clustering inside protected forested areas. The saplings grew back, their roots packing the soil together, and the rivers flowed narrower and faster- changing their course.

Seems very dramatic, but plausible, given the studies discussed earlier. Unfortunately, the effects that the wolves had on the Yellowstone ecosystem were overstated, and the conclusions drawn premature. It wasn’t the wolves’ fault- they were doing their job perfectly well. The fact was that after 70 years without their apex predator, conditions in Yellowstone had changed far too much for the wolves to rescue. It simply isn’t that easy to restore a heavily-eroded river to its former, fast-flowing state just by depending on tree growth.

This is a grim truth we all have to face: even if we return all top predators to their former ranges, we can’t expect them to reverse all the scarring left behind from their absence. And there are, sadly, very big scars. Some examples I pulled from a literature review on the effects of the loss of consumer species (Estes et al., 2011):

The reduction of lions and leopards in parts of sub-Saharan Africa led to an increase in the number of olive baboons. The baboons also became bolder without predators to fear, and came in increasing contact with humans. This also led to increasing infections by zoonotic diseases and parasites.

Because of their large size, whales hold on to huge amounts of carbon taken from their planktonic prey. When whale populations were devastated by whaling in the early 20th century, an estimated 105 million tons of carbon were returned to the atmosphere, contributing to global warming.

The loss of sharks and other large reef predators leads to greater numbers of coral-eating fishes, lower water clarity, and diminished coral reefs.

When harmful invasive species are introduced to ecosystems, invasions are much, much more successful in the absence of predators.

And of course, overall diversity is decreased when top predators are lost, because they no longer check the feeding habits of their prey, which can lead to a cascade of effects as described in the raccoon study.

Much of this damage is very hard to reverse. Indeed, since the Pleistocene, human eradication of megafauna (that is, large species of both herbivores and predators) have led to increasingly simplified and fragile ecosystems. The pristine earth we nature-lovers envision never really existed after humans entered the picture. We’re just… really good at changing things up, I guess you could say. In fact, you could argue that all the problems started when we lost our own ecology of fear- when we began crafting tools and weapons that allowed us to strike back at our predators, and became so confident in ourselves. Now we overgraze the planet.

I don’t mean to end this on a hugely sour note. There are some things we cannot ever change, but there are some things we can still fix, or at least begin to patch up. The research on the nonlethal effects of large carnivores is an important step in the process, because it will hopefully lead to more reintroductions like that of wolves in Yellowstone. Ecosystems need their top predators. And I do have hope that we can learn how to live with them- surely with all of our technology and ingenuity we can find a way to do that. Even if we are a little more scared.

One last thought, for this probably-too-serious Halloween post. We have been looking at fear from a very far-seeing ecological perspective, and it is fascinating how much one little emotion can change. Yet we can also zoom in a little more and ask how this fear impacts the lives of the individual animals it affects. In my series of articles on keeping animals in captivity, I mentioned that most researchers consider predation risk a form of acute stress- extreme when it occurs, but not long-lasting. However, when we look at the extreme changes in animal behavior when predators are merely hinted at, perhaps this is the wrong conclusion to draw.

In fact, field research on the effects of predator threat without actual predation (again, by doing things like muzzling, limiting access, or using trained dogs as predators) suggests that even mild exposure can cause chronic stress in prey animals, leading to poorer body condition, lower reproductive rates, and increased susceptibility to disease. This response to a predation threat is actually used to create animal models of PTSD. In other words, face a mouse with a caged cat just one time, and the effects will be traumatic and lingering.

What does this mean? Well, for starters, it could mean that PTSD-like symptoms are far more ‘natural’ than we may assume. Wild animals probably suffer from them all the time, unfortunately. Sheer terror is a very important aspect of animal behavior, with perceived danger perhaps being a much bigger factor than actual danger. But the flip side of this is perhaps by studying the so-called natural PTSD, we can understand how to better treat it in humans and captive animals. After all, even with the terror of predation hovering over them, wild animals still somehow manage to go about their daily lives with success. They still play, socialize, and mate. By studying their resilience, perhaps we can increase our own.